It is known to employ an inverting amplifier which is connected in a loop with a frequency selective element or branch, for example constituted by a quartz crystal, to constitute an oscillator. A resistor is connected between the input and output of the inverter to set the operating point of the oscillator. Such a circuit however is known to be disadvantageous, because the resistor, which may take the form of a transmission gate, typically has a poorly controlled resistance value. If there is no well controlled voltage bias for the transmission gate, the resistance presented by the transmission gate will vary significantly over the supply range. Poorly controlled feedback resistance leads to instability of the oscillator and can cause a long delay before the oscillator reaches a stable frequency of oscillation.
To alleviate these difficulties it has been proposed in EP-B-0315877 to provide a system of current mirrors that in essence provide weak inversion from the output back to the input of the main inverter. However, the circuit is still subject to process variations and requires the proper setting of the slope factor of a source follower. It also suffers from parasitic oscillations.